1. Field of the Invention
This invention relates to the field of borehole drilling, and particularly to systems and methods for controlling the direction of such drilling.
2. Description of the Related Art
Boreholes are drilled into the earth in the petroleum, gas, mining and construction industries. Drilling is accomplished by rotating a drill bit mounted to the end of a xe2x80x9cdrill stringxe2x80x9d; i.e., lengths of pipe that are assembled end-to-end between the drill bit and the earth""s surface. The drill bit is typically made from three toothed cone-shaped structures mounted about a central bit axis, with each cone rotating about a respective axle. The drill bit is rotated about its central axis by either rotating the entire drill string, or by powering a xe2x80x9cmud motorxe2x80x9d coupled to the bit at the bottom end of the drill string. The cones are forced against the bottom of the borehole by the weight of the drill string, such that, as they rotate about their respective axles, they shatter the rock and thus xe2x80x9cdigxe2x80x9d as the drill string is turned.
Boreholes are frequently drilled toward a particular target, and thus is it necessary to repeatedly determine the drill bit""s position. This is typically ascertained by placing an array of accelerometers and magnetometers near the bit, which measure the earth""s gravity and magnetic fields, respectively. The outputs of these sensors are conveyed to the earth""s surface and processed. From successive measurements made as the borehole is drilled, the bit""s xe2x80x9cpresent positionxe2x80x9d (PP) in three dimensions is determined.
Reaching a predetermined target requires the ability to control the direction of the drilling. This is often accomplished using a mud motor having a housing which is slightly bent, so that the drill bit is pointed in a direction which is not aligned with the drill string. To effect a change of direction, the driller first rotates the drill string such that the bend of the motor is oriented at a specific xe2x80x9ctoolfacexe2x80x9d angle (measured in a plane orthogonal to the plane containing the gravity vector (for xe2x80x9cgravity toolfacexe2x80x9d) or earth magnetic vector (for xe2x80x9cmagnetic toolfacexe2x80x9d) and the motor""s longitudinal axis). When power is applied to the motor, a curved path is drilled in the plane containing the longitudinal axes.
One drawback of this approach is known as xe2x80x9cdrill string wind-upxe2x80x9d. As the mud motor attempts to rotate the drill bit in a clockwise direction, reaction torque causes the drill string to tend to rotate counter-clockwise, thus altering the toolface away from the desired direction. The driller must constantly observe the present toolface angle information, and apply additional clockwise rotation to the drill string to compensate for the reaction torque and to re-orient the motor to the desired toolface angle. This trial and error method results in numerous xe2x80x9cdog legxe2x80x9d corrections being needed to follow a desired trajectory, which produces a choppy borehole and slows the drilling rate. Furthermore, the method requires the use of a mud motor, which, due to the hostile conditions under which it operates, must often be pulled and replaced.
A system and method of drilling directional boreholes are presented which overcome the problems noted above. The invention enables a desired drilling trajectory to be closely followed, so that smoother boreholes are produced at a higher rate of penetration.
The invention employs a controllable drill bit, which includes one or more drilling surfaces which are dynamically positionable in response to respective command signals. Instrumentation located near the bit measures present position when the bit is static, dynamic toolface and drilling surface position information when the bit is rotating, and stores a desired trajectory. This data is processed to determine the error between the present position and the desired trajectory, and the position of one or more of the bit""s drilling surfaces is automatically changed as needed to make the bit dig in the direction necessary to reduce the error.
The controllable drill bit is preferably made from three cone assemblies, each of which includes a cone and an eccentric cam that rotate about a common axis. In response to a command signal, the cam is either locked to the cone to cause concentric rotation of the cone, or locked to the axle to cause eccentric rotation of the conexe2x80x94which causes the bit to dig in a preferred direction.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.